KLK7 (hK7, or stratum corneum chymotryptic enzyme (SCCE), Swissprot P49862) is a S1 serine protease of the kallikrein gene family displaying a chymotrypsin like activity. KLK7 is mainly expressed in the skin and appears to play an important role in skin physiology (Egelrud. 1993. Purification and preliminary characterization of stratum corneum chymotryptic enzyme: a proteinase that may be involved in desquamation. J. Invest. Dermatol. 101, 200-204; Skytt et al. 1995. Primary substrate specificity of recombinant human stratum corneum chymotryptic enzyme. Biochem Biophys Res Commun 211, 586-589; Yousef et al. 2000. The KLK7 (PRSS6) gene, encoding for the stratum corneum chymotryptic enzyme is a new member of the human kallikrein gene family—genomic characterization, mapping, tissue expression and hormonal regulation. Gene 254, 119-1281).
KLK7 is involved in the degradation of the intercellular cohesive structure in cornified squamous epithelia in the process of desquamation. The desquamation process is well regulated and delicately balanced with the de novo production of corneocytes to maintain a constant thickness of the stratum corneum. In this regard, KLK7 is reported to be able to cleave the corncodesmosomal proteins corneodesmosin and desmocollin 1 (Simon et al. 2001. Refined characterization of comeodesmosin proteolysis during terminal differentiation of human epidermis and its relationship to desquamation. J. Biol. Chem. 276, 20292-20299; Caubet et al. 2004. Degradation of corneodesmosome proteins by two serine proteases of the kallikrein family, SCTE/KLK5/hK5 and SCCE/KLK7/hK7. J. Invest. Dermatol. 122, 1235-1244; Brattsand et al. 2005. A proteolytic cascade of kallikreins in the stratum corneum. J. Invest. Dermatol. 124, 198-203. In addition, it has been shown that the two lipid processing enzymes β-glucocerebrosidase and acidic sphingomyelinase can be degraded by KLK7 (Hachem et al. 2005. Sustained serine proteases activity by prolonged increase in pH leads to degradation of lipid processing enzymes and profound alterations of barrier function and stratum corneum integrity. J. Invest. Dermatol. 125, 510-520). Both lipid processing enzymes are co-secreted with their substrates glucosylceramides and sphingomyelin and process these polar lipid precursors into their more non-polar products e.g. ceramides, which are subsequently incorporated into the extracellular lamellar membranes. The lamellar membrane architecture is critical for a functional skin barrier. Finally, KLK7 has been shown to activate the pro-inflammatory cytokine Pro-interleukin-1β (IL-1β) (Nylander-Lundqvist & Egelrud. 1997. Formation of active IL-1β from pro-IL-1β catalyzed by stratum corneum chymotryptic enzyme in vitro. Acta Derm. Venereol. 77, 203-206).
Several studies link an increased activity of KLK7 to inflammatory skin diseases like atopic dermatitis, psoriasis or Netherton syndrome. An increased KLK7 activity might lead to an uncontrolled degradation of corneodesmosomes resulting in a miss-regulated desquamation, an enhanced degradation of lipid processing enzymes resulting in a disturbed lamellar membrane architecture or an uncontrolled (in)activation of the pro-inflammatory cytokine IL-1β. It has previously been demonstrated that this could lead to an impaired skin barrier function and inflammation (WO 2004/108139).
The KLK7 activity is controlled on several levels. Various factors might be responsible for an increased KLK7 activity in inflammatory skin diseases. Firstly, the amount of protease being expressed might be influenced by genetic factors. Such a genetic link, a polymorphism in the 3′-UTR in the KLK7 gene, was recently described (Vasilopoulos et al. 2004. Genetic association between an AACC insertion in the 3′UTR of the stratum corneum chymotryptic enzyme gene and atopic dermatitis. J. Invest. Dermatol. 123, 62-66.). The authors hypothesis that the described 4 base pair insertion in the 3′-UTR of the kallikrein 7 gene stabilizes the KLK7 mRNA and results in an overexpression of KLK7. Secondly, since KLK7 is secreted via lamellar bodies to the stratum corneum extracellular space as zymogen and it is not able to autoactivate, it needs to be activated by another protease e.g. KLK5 (Caubet et al. supra). Uncontrolled activity of such an activating enzyme might result in an over activation of KLK7. Thirdly, activated KLK7 can be inhibited by natural inhibitors like LEKTI, ALP or elafin (Schechter et al. 2005. Inhibition of human kallikreins 5 and 7 by the serine protease inhibitor lympho-epithelial Kazal-type inhibitor (LEKTI). Biol. Chem. 386, 1173-1184; Franzke et al. 1996. Antileukoprotease inhibits stratum corneum chymonyptic enzyme—Evidence for a regulative function in desquamation. J. Biol. Chem. 271, 21886-21890). The decreased expression or the lack of such inhibitors might result in an enhanced activity of KLK7.
It has been found that mutations in the spink gene, coding for LEKTI, are causative for Netherton syndrome (Descargues et al. 2005. Spink5-deficient mice mimic Netherton syndrome through degradation of desmoglein 1 by epidermal protease hyperactivity. Nat. Genet. 37, 56-65) and a single point mutation in the gene is linked to atopic dermatitis (Walley et al. 2001. Gene polymorphism in Netherton and common atopic disease. Nat. Genet. 29, 175-178; Nishio et al. 2003. Association between polymorphisms in the SPINK5 gene and atopic dermatitis in the Japanese. Genes Immun. 4, 515-517), Finally, another level of controlling the activity of KLK7 is the pH. KLK7 has a neutral to slightly alkaline pH optimum and there is a pH gradient from neutral to acidic from the innermost to the outermost layers in the skin. Environmental factors like soap might result in a pH increase in the outermost layers of the stratum corneum towards the pH optimum of KLK7 thereby increasing the KLK7 activity.
The hypothesis that an increased activity of KLK7 is linked to inflammatory skin diseases is supported by the following studies: Firstly, Netherton syndrome patients show a phenotype dependent increase in serine protease activity, a decrease in corneodesmosomes, a decrease in the lipid processing enzymes β-glucocerebrosidase and acidic sphingomyelinase, and an impaired barrier function (Descargues et al. 2006. Corneodesmosomal cadherins are preferential targets of stratum corneum trypsin-and chymotrypsin-like hyperactivity in Netherton syndrome. J. Invest. Dermatol. 126, 1622-1632; Hachem et al. 2006. Serine protease activity and residual LEKTI expression determine phenotype in Netherton syndrome. J. Invest. Dermatol. 126, 1609-1621.). Secondly, a transgenic mice overexpressing KLK7 shows a skin phenotype similar to that found in patients with atopic dermatitis (Hansson et al. 2002. Epidermal Overexpression of Stratum Corneum Chymotryptic Enzyme in Mice: A Model for Chronic Itchy Dermatitis. J. Invest. Dermatol. 118, 444-449; Ny & Egelrud. 2003. Transgenic mice over-expressing a serine protease in the skin: evidence of interferon gamma- independent MHC Il expression by epidermal keratinocytes. Acta Derm. Venereol. 83, 322-327; Ny & Egelrud. 2004. Epidermal hyperproliferation and decreased skin barrier function in mice overexpressing stratum corneum chymotoptic enzyme. Acta Derm. Venereol. 84, 18-22). Thirdly, in the skin of atopic dermatitis and psoriasis patients elevated levels of KLK7 were described (Ekholm & Egelrud. 1999. Stratum corneum chymotryptic enzyme in psoriasis. Arch. Dermatol. Res. 291, 195-200). Therefore, KLK7 is considered to be a target for the treatment of inflammatory skin diseases like atopic dermatitis, psoriasis or Netherton syndrome and there is a need for specific inhibitors thereof.
As patients suffering from Netherton syndrome have a severely impaired skin barrier, also topical administration of therapeutically active compounds will result in systemic exposure of the compounds to the patient. Accordingly, there is a need to identify inhibitors selective for skin proteases which can be used in the treatment of Netherton syndrome without risking systemic effects through unwanted systemic inhibition of proteases.
KLK7, KLK5, and KLK14 are believed to be part of a proteolytic cascade in the stratum corneum layer of human skin (Brattsand et al. 2005. A proteolytic cascade of kallikreins in the stratum corneum. J Invest Dermatol 124, 198-203).
Accordingly, it would be beneficial to identify inhibitors active not only on KLK7, but also on KLK5 and KLK14.
WO 2004/108139 describes certain substituted benzoxazinone and thienoxazinone compounds as inhibitors of KLK7, but fails to report any selectivity data for the described compounds.